1. Field of the Invention
The instant disclosure relates to a method for operating an automatic handling system, and more particularly, to a method for operating an automatic handling system that is applied to many wafer processing apparatuses.
2. Description of Related Art
In semiconductor manufacturing, lots of wafers have to be subjected to a fabrication process with multiple steps so as to be manufactured into desired semiconductor devices. In order to obtain profit, all the semiconductor manufacturers wish to reduce the time required for manufacturing lots of wafers, thereby increasing the yield of the semiconductor fabrication plant. There are many factors to influence the time required for manufacturing the wafers. In addition to the time required for manufacturing wafers on the respective fabrication machine tools, the time for transporting the wafers among different fabrication machine tools is also another critical factor.
The traditional transport of wafers is achieved by manpower. In this case, operators push vehicles in which wafers are carried back and forth between different fabrication machine tools. However, with the continuous advancement of the semiconductor manufacturing process, the size of wafer is increased from 6″, 8″ to 12″. As a result, the size of a wafer carrier for carrying wafers is increased accordingly. For example, a semiconductor plant often utilizes front opened unified pods (FOUP). When a FOUP is loaded with 25 pieces of wafers, the total weight of the wafer carrier is up to 8 kilograms, so that the traditional transport by manpower is not feasible any more. Moreover, taking the yield or other factors into consideration, it is necessary to provide a wafer transport system with high rate and efficiency, whereby the yield of a semiconductor plant can be increased. Therefore, an automatic transport system is developed for this purpose.
A common automatic transport system includes a plurality of overhead transport rail sets. A plurality of sets of fabrication machine tools is provided below each of the overhead transport rail sets. Each of the overhead transport rail sets defines a bay. The fabrication machine tools disposed in the same bay will be closely related to each other. The transport rail sets are provided with a plurality of overhead hoist transport vehicles (OHT vehicles) or called “vehicles” for short. The vehicles move on the overhead transport rail sets, whereby the wafer carrier can be transported from one fabrication machine tool to another fabrication machine tool. Further, each of the overhead transport rail sets is connected with another overhead transport rail set, so that the vehicles can move into different overhead transport rail sets to thereby transport the wafer carrier into the fabrication machine tool of another bay.